Response of tree growth and water use efficiency to climate and nitrogen deposition in a temperate deciduous forest in the northeastern U.S

Background/Question/Methods

Nitrogen (N) deposition and changing climate patterns in the northeastern U.S. can influence forest productivity through effects on plant nutrient relations and water use efficiency, defined as the ratio between CO₂ assimilation (A) and water loss via transpiration. To assess this impact, we considered the unique manipulation experiment at Harvard Forest, where N soil amendments has been carried out for the last 20+ years.  Within the forest, three plots were established: high (150 kg/ha/yr), low (50 kg/ha/yr), and control (ambient N inputs only). We sampled increment cores from the dominant tree species Quercus velutina in order to measure ring widths and stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions.  We evaluated the effects of climate and N deposition on tree growth and δ¹³C -derived iWUE (i.e., the ratio between A and stomatal conductance, gs). Response ratios were used to assess effects of N fertilization on basal area increment (BAI) and iWUE between treatment and control trees, while linear mixed effect models were used to evaluate the combined effects of N fertilization and monthly growing season climate on BAI and iWUE. We hypothesized that trees would exhibit a pronounced yet transient increase in increment growth and iWUE with the onset of N fertilization, and more subtle interannual fluctuations from variations in climate.

Results/Conclusions

Fertilized trees exhibited a more pronounced and sustained growth enhancement relative to control trees, with the low- and high- N treatments responding similarly. Trees growing in all three plots exhibited improved iWUE over the study period (1984-2011). Trends in iWUE in control trees were primarily driven by changes in stomatal conductance, consistent with recent analyses indicating region-wide improvements in iWUE related to rising atmospheric CO₂. Fertilized trees exhibited a more pronounced enhancement in iWUE over the control trees which was attributed to a stimulation in photosynthesis, with the high-N trees showing the greatest increase. These differences mirrored patterns observed for foliar N concentration where trees in the fertilized plots had significantly elevated foliar N concentrations relative to pre-treatment levels. Ongoing analyses using linear mixed effects models indicate that cumulative N fertilization may have a predominant effect on BAI and iWUE, with climate signals (early summer precipitation and mid-summer temperature) exerting a secondary effect. Combined, these results suggest that sustained ambient N deposition in N-limited Northeastern forests may enhance the forest-C sink thus mitigating some of the negative effects of droughts forecasted for the region.